The present invention relates to a head for the linear dimension checking of pieces in machine tools or measuring machines, with a support structure, a movable arm-set including an arm carrying a feeler for contacting the piece to be checked, a bias device arranged between the support structure and the movable arm-set, a constraining system arranged between the support structure and the movable arm-set including a first surface with rotational symmetry and a second surface with rotational symmetry, both these surfaces with rotational symmetry being urged to contact each other by the force provided by the bias device and an electric detecting device, arranged at said first surface with rotational symmetry and second surface with rotational symmetry, for providing a signal depending on the reciprocal position of the surfaces with rotational symmetry.
Heads or contact detecting, or xe2x80x9ctouch triggerxe2x80x9d, probes, with movable arm-sets that carry feelers, are used in coordinate measuring machines and machine tools, more particularly machining centres and turning machines, for checking machined or being machined pieces, tools, machine tables, etc. The checkings can regard, for example, the position or the geometric dimensions of the pieces. In each of these probes, contact between the feeler and, for example, a piece is monitored by suitable devices that detect specific displacements of the movable arm-set with respect to a support structure, and control the reading of transducers associated with the machine slides, that provide measurement values with respect to a reference position or origin.
The known heads of the type mentioned at the beginning of the description are described in DD-A-214434. In these heads the detecting device comprises a disk made of an electric conducting material to which there is coupled the arm carrying the feeler and that defines the second annular surface and at least a pair of elements, shaped in the form of portions of an annulus, also made of an electric conducting material. Both elements are arranged on the support structure at the first annular surface and are separated by portions made from insulating material. The detecting device detects the closure or opening status of a circuit in which, in the absence of forces acting on the feeler, the second annular surface and the two elements are electrically connected in series.
The movable arm-set can accomplish, subsequently to contact between the feeler and the piece to be checked and depending on the reciprocal displacement direction between the head and the piece, translation displacements along the longitudinal axis (Z) of the head and rotational displacements about a contact point between the two annular surfaces. Owing to the fact that usually the displacements of the movable arm-set are limited, upon the rotational displacements there occur, in substance, lateral displacements of the feeler (in the X-Y plane).
These known heads are subject to some drawbacks, for example due to the fact that the first annular surface is made of two different materials. This may cause, besides manufacturing complexity, an anisotropic behaviour of the head, more specifically as a consequence of different thermal expansions. On this regard, the concept of anisotropy substantially consists in the fact that, as the direction of the transversal displacement of the feeler changes, the triggering of the detecting device takes place at different eccentricity values of the feeler with respect to the longitudinal geometric axis of the head.
Other drawbacks that are due to the fact that the first annular surface is made of two different types of material can consist, for example, in the interruptionxe2x80x94when the head is in the inoperative positionxe2x80x94of the electric connection between the disk and the two elements made of electrically conducting material or the failure to interrupt the connection as a consequence of the feeler displacing along a transversal direction. Obviously, this latter error in the operation may also cause irrepairable damage to the head and, in general, serious problems insofar as safety is concerned, because the contact signal sent by the head is generally utilized for controlling the stopping of the displacements of the machine slides.
Object of the present invention is to provide a head with limited isotropy errors with respect to transversal displacements of the feeler, constructional simplicity and a highly reliable detecting device. This and other objects are provided by a head of the type hereinbefore described in which the detecting device includes a first electrically resistive portion arranged in the support structure and a second electrically resistive portion arranged in the movable arm-set, said first and second resistive portions being substantially electrically connected in parallel in the condition according to which said first and second surfaces with rotational symmetry are in abutment.